By placing bins on top of the others eventually you just get a very dense and packed material that probably could even go anaerobic.
I just posted pictures of my new flow through bin http://forums.gardenweb.com/forums/load/verm/msg0422570210660.html.
What you are mentioning here is more of a stacking bin system. A stacking bin system can work but generally the top bin would not press on the contents of the bottom bin, it would just touch it so the worms would migrate upwards. The thing is the worms will decide where they will go and are probably staying put in the lower levels as there is enough food for them.

How slowly are you adding buckets? If there is still a substantial amount of food (including bedding) in the lower buckets, many of the worms will stay in the lower buckets.